The note is not lost. It just lives on the active version of each task. If you want to retain a copy of the note on each historical version of a task, you will have to copy and paste the note from the active version into the completed version. We may make this a user configurable setting in the future.

The reason this is important to me is because I recently accidentally deleted some notes with extensive note.

I'm not sure I understand 'active version of each task.
If I have a task to call someone and put several details like the phone number or price into the note, when I check the task off as completed, there is not longer an 'active version' , the only thing left is the archive of the task saying to call someone.
That means the note is lost.
Is this correct?

If you have a single task with a note, and you check it off, that note will remain and will not be deleted.

What I was talking about is repeating tasks. If you have a weekly repeating task with a note, the note will be on the activate version of the task, and if you go and look up an old completed copy of that repeating task, it will not have a note.

I raised this subject because it seemed like when I searched the archive there were no notes in any of the iterations. From what you are saying, if I've finished with a recurring task the last occurrence in the archive should have the note, right?

I deleted a recurring task, thinking that I could easily recover information in the notes from recently completed tasks. But as described above they are all empty and the current task has been deleted.

That's certainly not what I expected. Is there any way to recover this deleted information?